Hi, there!
Just checking into see how your bird is doing. Updates to these threads are very useful for people in the future experiencing the same problem.
You're are most certainly welcome! Very sorry you have to go through this, please keep us posted!
You may read up on this thread I made incase some other birds start showing symptoms.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
Not completely sure, but from the picture, it looks like your duckling may have developed perosis in the one leg, which can be a symptom of a niacin deficiency. Your best bet would be to have her taken to a vet. I should emphasize that she should be taken as soon as possible, if left untreated...